Which of the following lists two methods for freezing tissue?

• A. In a cryostat; In isopentane and liquid nitrogen
• B. In a freezer; In acetone
• C. In formalin; In water
• D. In ethanol; In dry ice

Answer: (A)

Freezing tissue for cryosectioning depends on turning the tissue temperature very quickly to prevent ice crystal formation that can distort cells and degrade morphology. Using a cryostat provides a cold environment in which tissue can be frozen and kept while you cut thin frozen sections for microscopy. A second well-established method is rapid freezing by immersing tissue in isopentane that has been chilled with liquid nitrogen; this produces an extremely fast temperature drop, yielding finer ice crystals and better preservation of cellular details and lipids. The other options involve fixatives or solvents that aren’t used for freezing in preparation for frozen sections, so they don’t represent freezing methods. This combination—freezing in a cryostat and rapid freezing in chilled isopentane with liquid nitrogen—reflects two common freezing approaches.